In the graphic arts and in motion pictures, it is often desirable to isolate a subject or element, in a photographic image, from its surrounding background in order to place it against a different background. The process of removing the background is called stripping in the graphic arts, and was once done with a pair of scissors or razor blade.
In the motion picture industry, when the desired background is not available, the subject is photographed against a colored backing which is subsequently replaced by a suitable background scene with a travelling matte compositing process such as described in U.S. Pat. No. 4,625,231.
There are occasions however, when the subject is not photographed against a background suitable for travelling matte compositing. In this case, the unwanted portions of the scene surrounding the subject are removed by hand, one frame at a time. This process is known as Rotoscoping and was once done by hand painting enlarged frames of the image. Today the stripping and hand painting is done on a color monitor using a computer, mouse and cursor, or equivalent. In either case the work is slow and tedious, and it is difficult, if not impossible, to properly reproduce full hair detail and subjects that are semitransparent.
Another process, called difference matting, photographs the scene twice, once with subject removed to produce a Clear Frame, and a second time with subject in place. The Clear Frame is an image frame in which all subject matter has been cleared from the set. By subtracting the Clear Frame from the subject frame, one obtains a zero signal for every pixel in the background area. When the difference at a given pixel deviates from zero, that pixel contains elements of the subject. This remainder is rejected in favor of retaining the subject. The subject then appears to have been photographed against a black backing. However, this technique does not remove background elements from semitransparent areas in the subject.